Pretty much as the title says. I have a NextCloud 13 install with the 3.2.1 Talk installed.
One of the users left so I deleted his account from Active Directory and from NextCloud. The Talk chat still persists. I then tried to delete the Chat from both the latest client for NextCloud Talk (1.2.0) and when that failed I attempted to do it via the actual web interface. I do not see an error. It just doesn’t do anything.
I just tried to reproduce your issue, but I could not.
Nextcloud Talk listens for user deletions within Nextcloud and automatically removes that user for any conversation where that user is participating and also every one-to-one conversations of that user.